This invention relates to mountings for cranes, and more particularly, to a retainer which limits the separation of the upper works of a crane from its base during the occurrence of an abnormal condition. This retainer may find particular application in use on marine crane mountings.
With the development in recent years of offshore oil drilling platforms anchored at sea, specialized luffing cranes have been installed on such platforms for handling loads both on the platforms and between the platforms and ships brought alongside. Such cranes are commonly called marine cranes and the revolving upper works of a conventional marine crane is supported on a swing circle mounted atop a pedestal fixed to the platform. Instances have recently occurred where such cranes while lifting and transferring cargo between a ship and a platform have been subjected to unusually large, dynamic loads peculiar to the nature of their use. For example, if cargo aboard a ship is hooked to a crane while the ship is in a turbulent or heavy sea, a dynamic load may be imposed on the crane by the action of the ship falling away from the platform, or by a vertical rise and fall of the ship in response to passing crests and troughs of the waves. The roll and pitch of the ship may also impose extreme dynamic load variations, particularly when the crane hoist line is caused to slacken and tighten in response to ship movement while the load is not fully airborne or free of the ship. The cargo then presents a rapidly varying load on the crane that may impose unusually large peak stresses. Severe and unpredictable load stresses can also develop from cargo catching on ship rails, hatches or other protrusions of a ship superstructure.
These various dynamic load variations can be accentuated when crane operators and other personnel use the equipment under conditions which require greater caution than that exercised. An operator may, for example, lower the boom tip to stay over a load moving outward from the crane as a ship falls away, and thus increase the boom radial distance beyond the rated capability of the equipment for the particular load, or conversely, he may allow the load to become improperly positioned with a shift of the ship, and then commence lifting under improper conditions. In the exigencies of such situations, these maneuvers of an operator may be carried out without properly checking the relation of the load to the crane rating, and parts may be stressed beyond limits for which the crane is designed. It also appears that at times an operator may simply overload his crane beyond its designed capacity to load or unload a cargo as quickly as possible.
The unpredictable, dynamic loads that lead to overloading of marine cranes, as contrasted to the basically static and pure lifting loads of land cranes, makes it desirable to have an arrangement that reduces the possibility of total separation of a crane upper works from its pedestal. This would minimize possible injury to personnel, loss of the crane and other property damage.
The prior art has disclosed a crane modification to counteract the effects of excessive loading of marine cranes in U.S. Pat. No. 4,011,955 issued Mar. 15, 1977, to Morrow Sr. et al for a "Sea Crane Tiedown." There, an annular collar is attached to a pedestal, and a ring-like device positioned beneath the collar is suspended by tension links depending from the frame of the crane upper works. This ring-like device is out of contact with both the collar and pedestal, and the entire structure is referred to as a "sea crane tiedown." Such structure is bulky and complex, and difficult to install. British Pat. No. 1,470,019 for an "Offshore Counterbalancing Crane" discloses hook rollers that are mounted on a bracket for rotating along the underside of a flange of a combination bull gear and roller path. This, however, is not for the purpose of providing retention of the upper works in case of an overload that could topple the upper works from the crane base.
Arrangements that limit the separation of an upper works from its supporting structure have also been provided for other types of material handling machines. One such device is disclosed in U.S. Pat. No. 1,929,397 issued Oct. 3, 1933, to Davidson for an "Excavating Apparatus." Davidson discloses radial lugs that extend under a projection along the circumference of an annular track. Another device is disclosed in U.S. Pat. No. 2,408,378 issued Oct. 1, 1946, to Davenport for a "Stabilizer Attachment For Cranes." Davenport shows a plurality of roller-carrying hangers that ride along an annular track during operation of the crane. None of these arrangements are entirely satisfactory, however, and the present arrangement has been developed to provide an improved means for limiting the separation of the upper works of a crane from its supporting structure during the occurrence of an abnormal condition.